Spectrographic multiplexers having an array of waveguides conventionally comprise a dispersive array of light guides connected to inlet and outlet waveguides via two star couplers. The field in an inlet waveguide is reproduced in the plane of the outlet waveguides when the optical path length difference between two adjacent waveguides in the array is equal to an integer number of incoming wavelengths. Varying wavelength causes the distribution of the field over the outlet waveguides to be moved in translation. Such a configuration thus serves to separate different wavelengths in space.
By way of example, such components are used as a 1 to N demultiplexer, as an N to 1 multiplexer, or as an N to N multiplexer with switching.
A particularly advantageous application for such components lies in the field of optical fiber telecommunications, for example with high rate transmission (e.g. in a receiver circuit), or in optical distribution networks (wavelength separator), or in optical switching devices (highly integrated high speed electronic "chips" or between computers or within a computer).
As a general rule, the spectrum response obtained in an outlet waveguide from such a component corresponds to coupling a Gaussian beam into a Gaussian waveguide, and is thus itself Gaussian.
However, a Gaussian spectrum response requires accurate control over emitted wavelengths, thus making it difficult to use in a system. Unfortunately, the existence of small fluctuations in the wavelength at which a laser emits (fluctuations due to temperature) makes it necessary to be able to use channels having broader spectrum responses (or to servo-control the laser, which is difficult to do and expensive).
Broadening the spectrum response also makes polarization independence easier to achieve. The shape of waveguides is highly critical in obtaining polarization independence (whatever method is used). Thus, by having a flat spectrum response, the power received over a channel is independent of polarization, even if its TE and TM peaks are slightly offset (offset due to shape being slightly inaccurate).
Several techniques have already been proposed for making a spectrograph having an array of waveguides that presents a square type spectrum response.
In particular proposals have been made in: "Phased-array wavelength demultiplexer with a flattened wavelength response", University of Delft, M. R. Amersfoort, et al., IEEE Electronic Letters, February 1995, Vol. 30, No. 4; and "Polarization-independent InP-based phased-array wavelength demultiplexer with a flattened wavelength response", University of Delft, L. H. Spiekman et al., ECOC 94; to enlarge the outlet waveguides so as to obtain truly multimode outlet waveguides. Such multimode waveguides can couple with the incident beam over a broader range of wavelengths.
However, such multimode waveguides do not enable the outlet of such a spectrograph having an array of waveguides to be coupled to other elements such as another phasar, an optical amplifier, etc.
The very poor coupling between a multimode waveguide and a monomode fiber makes such a component unusable in monomode fiber networks.
Such spectrographs can be used only with photodiodes disposed at the outlet from the multimode waveguides in order to detect output powers.
Proposals have also been made in "Arrayed-waveguide grating multiplexer with a flat spectral response" NTT, K. Okamoto and H. Yamada, Optics Letters, January 1995, Vol. 20, No. 1, to achieve a "(sinx)/x" type power distribution in the array by modifying both the power distribution at the inlet to the waveguide array and the phase shifts of the waveguides in the array.
No practical embodiments have yet been achieved using that structure, for the following reasons: secondary power lobes are obtained in the array, making it necessary firstly to modify the mode shape in the inlet guide, which shape must be square, and secondly to provide additional phase shifting on the waveguides which correspond to the secondary lobes.
It will be understood that such a technique which involves modifying the inlet power shape and the phase shifting in the array of waveguides is difficult to implement, with poor control over manufacturing parameters giving rise very quickly to degraded component performance.
In particular, the phase shifting corresponding to the secondary lobes must be accurately controlled, since a positioning error on a waveguide concerning the additional phase shifting degrades the spectrum response very greatly and makes the phasar unusable.